Magnetic recording media have been widely used as recording tapes, video tapes, floppy discs or the like. In general, a magnetic recording medium is constituted by laminating a magnetic layer on a support, and laminating a back layer on the side thereof opposite to the magnetic layer as needed for a tape-shaped medium. For a disk-shaped medium, magnetic layers are laminated on both sides of a support.
Then, a magnetic layer of a coating type magnetic recording medium is generally formed by applying onto a support a dispersion of a ferromagnetic powder in a binder to which a lubricant and an abrasive, and carbon as needed, are added.
In recent years, in order to increase the output of a magnetic layer, it has been proposed to reduce the thickness of the magnetic layer. For this purpose, a magnetic recording medium in which an intermediate layer is provided between a support and a magnetic layer has been proposed.
Further, in a vapor deposition type magnetic recording medium, a magnetic film is formed on a support by vacuum deposition. A metal or alloy mainly comprising cobalt is vapor deposited as a magnetic substance in an atmosphere of oxygen, and a protective film and a lubricant film are formed on the magnetic film as needed.
With respect to the magnetic recording medium thus obtained, an audio tape for music recording and reproduction is required to have higher ability to reproduce an original sound, a video tape is required to be excellent in ability to reproduce an original image, and a backup tape or disc for computers is required to be excellent in storage stability, to be satisfactory in durability and to cause no data loss.
In order to allow the magnetic recording medium to have excellent electromagnetic characteristics and to ensure durability so as to meet various requirements as described above, heightening the Hc of magnetic substances, increasing orientation, thinning coated layers, developments of protective layers for magnetic layers, and developments of lubricants for reducing the coefficient of friction of magnetic layers/back layers have been conducted.
On the other hand, on the side of recording and reproduction apparatus, recording wavelengths are being shortened and tracks of magnetic recording heads are being narrowed as means for increasing recording capacity per unit area.
Further, magnetic heads which work with electromagnetic induction as the principle of operation (induction type magnetic heads) come to have a limitation in the use thereof in a higher density recording and reproduction region. That is to say, in order to obtain large reproduction output, it is necessary to increase the turns of a coil of a reproduction head. However, the inductance increases, and the resistance at high frequency increases, resulting in a reduction in reproduction output.
In recent years, reproduction heads which work with MR (magneto-resistance) as the principle of operation have been proposed and have come to be used in hard discs, etc. In the MR head, reproduction output of several times that of the induction type magnetic head is obtained, and no induction coil is used, so that instrument noises such as impedance noises are extremely reduced. It has become therefore possible to obtain a high S/N ratio by decreasing noises of a magnetic recording medium. In other words, when the magnetic recording medium noises which have hitherto been hidden behind the instrument noises are decreased, good recording and reproduction can be performed, thereby being able to drastically improve high density recording characteristics.
In the meantime, in order to provide a magnetic recording medium for high density recording remarkably improved in electromagnetic characteristics, particularly in high density recording characteristics, JP-A-2003-30813 has proposed a magnetic recording medium comprising a support having provided thereon a magnetic layer mainly containing a ferromagnetic powder and a binder, wherein the ferromagnetic layer is a hexagonal ferrite powder having an average tabular diameter of 15 to 40 nm, the perpendicular factor of the squareness ratio SQ of the magnetic layer is from 0.1 to 0.55, the specific surface area and the total pore volume of the magnetic recording medium itself measured by a nitrogen absorption method are from 0.1 to 50 m2/g and from 0.001 to 1 ml/g, respectively, and the magnetic layer thickness is 0.02 to 0.3 μm.
However, the thickness of the magnetic layers described in the examples of JP-A-30813 is as thick as 0.2 μm. In such thick magnetic layers, the problem has been encountered that the dispersibility of the ferromagnetic powders is deteriorated to cause the generation of aggregates, thereby increasing noises. (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)